The present invention relates to a coma compensating element for compensating coma of a high NA optical system employed in an optical pickup for an optical disc having a relatively high data recording density, and an optical system for such an optical disc employing the coma compensating element.
An optical system for an optical pickup is generally provided with a light source for emitting a laser beam, an objective lens for converging the light beam emitted by the light source onto an optical recording medium such as an optical disc, and a light receiving unit for receiving the light beam reflected by the data recording surface of the optical disc, and outputting signals. The objective lens is generally a single-element lens having aspherical surfaces. Alternatively, the object lens may consist of a plurality of lens elements each having spherical surfaces. The objective lens is generally designed so that spherical aberration and coma are well suppressed, and the laser beam is converged to form a diffraction-limited beam spot.
If the optical axes of both surfaces of the aspherical single-element lens are decentered (i.e., parallelly shifted from each other), or at least one lens element within a combination lens is decentered with respect to the optical axis of the remaining lens elements, even if the light beam incident on the objective lens coincides with the optical axis of the objective lens, coma is generated. In particular, for a disc drive using a disc having a relatively high data recording density, the objective lens should have a high NA (numerical aperture). Such a high NA objective lens is sensitive to the decentering of the surfaces. That is, with a slight decentering, significantly large coma is generated. Generally, the objective lens whose surfaces are aspherical is formed using metal molds. Specifically, two metal molds are used for both aspherical surfaces, respectively. Since a certain clearance is required between the two molds for manufacturing procedure, it is unavoidable that the two surfaces are decentered (i.e., shifted in a direction perpendicular to the optical axes thereof) with respect to each other, and the thus formed lens provides coma (i.e., decentering coma).
When an objective lens that provides the above-described decentering coma, or an objective lens whose abaxial coma is not completely compensated due to balance among various aberrations is used, it is necessary to align the direction of the beam incident on the objective lens and the optical axis of the objective lens precisely. Therefore, it is very difficult to adjust the arrangement of the optical elements of such an optical system. If the objective lens provides coma, and the adjustment (alignment) is insufficient, the laser beam cannot be sufficiently converged and a beam spot having the desired radius cannot be formed. In such a case, data may not be written/read correctly.
Further, in an optical pickup used for optical discs with cover layers having different thickness such as a DVD (digital versatile disc) and a CD (compact disc), and/or used for optical discs requiring different beam spot sizes, light beams having different wavelengths are used in order to realize different NA using the same objective lens. In such a case, the quantity of coma is different when the incident beam is inclined with respect to the optical axis of the objective lens. Therefore, it is impossible to design an objective lens which is capable of compensating coma for every disc.